The powder metallurgy industry has developed iron-based powder compositions that can be processed into integral metal parts having various shapes and sizes for uses in the automotive and electronics industries. One processing technique for producing the parts from the base powders is to charge the powder into a die cavity and compact the powder under high pressures. The resultant green part is then removed from the die cavity and sintered.
To avoid excessive wear on the die cavity, lubricants are commonly used during the compaction process. Lubrication is generally accomplished by either blending a solid lubricant powder with the iron-based powder (internal lubrication) or by spraying a liquid dispersion or solution of the lubricant onto the die cavity surface (external lubrication). In some cases, both lubrication techniques are utilized.
Lubrication by means of blending a solid lubricant into the iron-based powder composition has disadvantages. First, the lubricant generally has a density of about 1-2 g/cm.sup.3, as compared to the density of the iron-based powder, which is about 7-8 g/cm.sup.3. Inclusion of the less dense lubricant in the composition lowers the green density of the compacted part. Second, internal lubricants are generally not sufficiently effective for reducing the ejection pressures when manufacturing parts having part heights (the minimum distance between the opposing punches in the press) in excess of about 1-2 in. (2.5-5 cm). Finally, when the particles of internal lubricant burn off during sintering, pore spaces can be left in the compacted part, providing a source of weakness for the part.
The use of external, die wall lubricants has generally taken the form of aqueous dispersions of the solid lubricant. The use of these lubricant compositions can reduce or eliminate the need for an internal lubricant, but problems also accompany external lubrication techniques. First, the film thickness within the die cavity has a tendency to vary, and the lubricant dispersion is known to drip out of the die cavity during processing. Also, aqueous dispersions are a source of rust formation on the die cavity. Finally, various commercially available external lubricant compositions are not necessarily sufficient to adequately lower ejection forces, especially at higher compaction pressures.
According to the present invention, there is provided an external lubricant, which avoids the problems of reduced green density and sintered strength, but which provides uniform lubricity to the die wall and minimizes ejection forces.